One embodiment of a centrifugal separator is shown in U.S. Pat. No. 4,776,833 of the present inventor which is issued on 11 Oct. 1988. This patent shows a centrifugal bowl of the above type in which there is an outer liner of metal upon which is cast an inner liner of a plastics material which is formed on the inner surface to define grooves extending peripherally around the bowl.
In order to communicate water into the bowl from the outside surface of the peripheral wall through the peripheral wall into the base of the grooves, a plurality of holes are drilled which extend angularly around the bowl so as to tend to inject the water in an angular direction around each groove. One technique for the formation of this angularly extending hole or duct is shown in the patent in which a portion of the peripheral wall is punched inwardly to form an indent which is substantially triangular shaped in plan view. Subsequently the liner is cast on the inside surface of the punched bowl and then the hole is drilled along one side of the triangular shaped punched indent breaking out through a second side of the triangular indent through the peripheral wall into the plastics material of the liner and through the plastics material of the liner to the inner surface for communicating the water from the indent into the bowl.
It is stated in the patent that the cast plastics material is permanently attached to the shell by the natural bonding effect and by the punched indents or depressions. In this embodiment however the punched indents simply form a roughening of the surface of the bowl to assist in the adhesive or frictional effect holding the liner attached to the shell.
However in practice is has been found that the natural bonding effect and the simple frictional effect of the punched depressions is insufficient to hold the liner permanently attached to the bowl, particularly bearing in mind the very large forces that are involved in the rotation of the bowl and also bearing in mind the different coefficients of thermal expansion of the plastics material and the shell which tend to cause a separation of the liner from the shell.
In practice, therefore, it has been necessary until now to weld onto the inside surface of the bowl a number of metal bars which assist in holding the liner properly attached to the shell since they extend sufficiently deeply into the liner to prevent separation from the shell.
However this technique is inefficient and undesirable since it requires the addition to the bowl of further material which increases the weight and also is an additional process causing further expense. Furthermore the bars themselves are a simple and inefficient technique for attachment to the liner and therefore the size of the bars has to be relatively large to obtain the attachment necessary.